Single-pole, single-throw (SPST) PIN diode switches provide a convenient way of coupling a single input signal to one of a plurality of output terminals. Such a flexibly configurable topology can be used, for example, in plasma processing systems in which one high-power radio-frequency (RF) generator can be used as an energy source for a plurality of plasma chambers or for different electrodes of the same plasma chamber. For RF generators feeding plasma processing systems, the transmitted power can be very high—as much as 5 kW or more. Furthermore, the reliability and stability of the switch can impact the performance of plasma processing equipment.
PIN diode SPST switches are completely electronic and, therefore, inherently present various feedback paths between the output terminals of the switch. Many applications require at least about 40 dB of signal isolation between output ports serviced by the same input port. A circuit that would combine the versatility of high power transmitted power with advanced isolation characteristics and stability would have a multitude of applications.
FIG. 1 shows a typical configuration of a PIN diode SPST (“switch”) 100. When switch 100 is “closed” (configured to allow current to flow), PIN diode 105 is forward biased, presenting a very low impedance to the RF signal passing from input terminal 110 to the output terminal 115. DC voltage sufficient to forward bias PIN diode 105 is applied to control port 120 and creates DC current flowing through DC-conducting and RF-isolating element 125, PIN diode 105, and DC-conducting and RF-isolating element 130. DC-conducting and RF-isolating elements 125 and 130 have low DC impedance for the biasing current and high RF impedance at their points of connection with PIN diode 105. When switch 100 is closed, PIN diode 135 is reverse biased, presenting very high impedance to the RF signal and having negligible shunting action to the output of the switch.
When switch 100 is “open” (configured to prevent current from flowing), PIN diode 105 is reverse biased, presenting very high impedance to the RF signal passing from input terminal 110 to output terminal 115. But the junction capacitance of PIN diode 105 allows a significant portion of the coupled microwave signal to pass through switch 100 when switch 100 is in the “open” position. In the very-high-frequency (VHF) range, the junction capacitance can limit isolation between input terminal 110 and output terminal 115 to only 20 to 25 db. Forward biased PIN diode 135 provides a low impedance shunt from output terminal 115 to ground 140, improving isolation to at least 40 db. The bias of PIN diode 135 is controlled by control port 145.
Capacitors 150, 155, 160, and 165 are all blocking capacitors, meaning they have low impedance at the operational frequency and do not affect the transmission and isolation properties of switch 100. In VHF frequency range, lumped circuit elements (multi-turn coils) are typically used as the DC-conducting and RF-isolating elements 125 and 130. But in the configuration shown in FIG. 1, the full RF voltage is applied to the coils. At a transmitted power level of a few kilowatts, this voltage can reach many hundreds of volts. In VHF range, such a high voltage usually results in considerable thermal problems for the multi-turn coils. To handle such a high RF voltage, the coils need a very low loss factor (hence big size) and require a complex cooling system. Another drawback of the coils is low temperature stability and long-term mechanical instability if expensive mechanical constructions are not used. These factors limit using lumped circuit elements for high-power and high-reliability systems.
One of the requirements for DC-conducting and RF-isolating elements 125 and 130 is high RF impedance at operational frequency. Some prior-art high-power PIN diode switches are implemented using a distributed, constant-transmission-circuit, quarter-wavelength, resonant transmission line. This type of RF-isolating element is used in narrow-band applications, which is typically the case with plasma processing systems. The impedance of the shorted-at-the-end, quarter-wavelength, resonant transmission line at resonant frequency theoretically should be infinite, but due to the finite resistance of the material of which the transmission line is made and dielectric losses in the isolation, the actual impedance can be considerably low. DC-conducting and RF-isolating elements 125 and 130 are connected in parallel to input terminal 110 and output terminal 115, and the low input impedance of DC-conducting and RF-isolating elements 125 and 130 means high RF energy loss in those elements.
Transmission lines can be realized using microstrip technology on thermally conductive substrates. This allows dissipating sufficient power in the DC-conducting and RF-isolating elements and operating at higher transmitted power. Using ceramic substrates provides high stability and reliability for the switch. But switches employing quarter-wavelength, resonant transmission lines have significant drawbacks. In VHF frequency applications, the length of the quarter-wavelength segments is large compared to the remainder of the circuit. Therefore, the size of the housing and the length of the conductors for the switch are increased compared to other switches.
To decrease the size of the housing for the quarter-wavelength circuit, the folded stripline shape is used frequently. FIG. 2A shows one example of a quarter-wavelength circuit 200 in which the stripline 205 has a meandering (snake-like) shape. Stripline 205 is disposed onto thermoconductive substrate 210, which is thermally attached to heat sink 215. Other elements of the PIN diode switch (the PIN diode itself, the capacitors, and the RF and bias-control ports) are also disposed on the same thermoconductive substrate 210, but those elements are not shown in FIG. 2A for simplicity.
The isolation properties of the folded stripline 205 deteriorate when the distance between adjacent sections of the folded stripline 205 becomes less than or equal to the width of the folded stripline 205. The reason for this is that the configuration of the magnetic field of the folded stripline 205 is different from that of a straight stripline. RF currents in adjacent sections of a folded stripline flow in opposite directions. This is shown schematically in the cross-section A-B of FIG. 2B, in which the directions of current flow are shown above the sections of folded stripline 205 as circled crosses (into the page, away from the reader) and circled dots (out of the page, toward the reader). Adjacent sections of folded stripline 205 are coupled magnetically, and that magnetic coupling “M” results in the partial cancellation of the magnetic fields of adjacent sections. As a result, the characteristic impedance of the transmission line decreases, and the electrical length of the line decreases, requiring that the physical length of the stipline be increased to satisfy the quarter-wavelength, resonant conditions. All those changes increase RF energy losses in the line. Those effects can impose a practical limit on the extent to which the size of the stripline can be compacted.
Although the technical solutions of the prior art discussed above provide significant improvements in the art, there remains an ongoing need for further improvements in the design of high-power microwave switches, particularly for very high power applications involving plasma processing with transmitted power up to 5 kW in the VHF frequency range.